Most people would probably feel pretty queasy about living atop — or even near — an old graveyard. Imagine if you lived above the mass graves of thousands of diseased individuals who were unceremoniously buried during the Great Plague of 1665 to 1666. That's the ghoulish reality that many unwitting residents of the greater London area must come to terms with, thanks to a new map that details the extent of that horrific scourge, reports Atlas Obscura.

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An estimated 100,000 people, possibly as much as a quarter of London's population, died during those ghastly two years. Death was so prevalent that bodies were heaped together into immense pits in unconsecrated ground and buried. These so-called "plague pits" were initially intended to be left undisturbed, but over the years the city of London has been built over them, concealing this gruesome legacy.

Though many of the plague pits were mostly forgotten, occasionally a new building project serves as a grisly reminder, unearthing layers of old bones. For instance, the construction of a Crossrail station near Liverpool Street station unintentionally exhumed around 4,000 skeletons that were identified as centuries-old plague victims.

The graves are so widespread that online history magazine and guide, Historic UK, has made a map of their known locations. Pits are marked with a skull and crossbones, scattered all over London:

Plague mapScreenshot of Historic UK's plague pit map. (Photo: Historic UK)

Though dotted with gravesites, the map is almost certainly incomplete. Many parks, some that are popular picnic spots, have since been built over plague pits. Plague pits lie beneath supermarkets, churches and office buildings.

The map is a grim reminder of London's dark history with the plague. It reminds us that as much as we would like to bury the past, the past still lurks, haunts. For Londoners, it quite literally skulks beneath every footstep.